Physical Light Units
Lux, lumens, candela and nits
Also known as: Lux lumens candela · Light units Unreal · Light units Unity · Nits · Photometric units · Light intensity units
Definition
Physical light units are real-world measures used to set light intensity in engines. Lumens measure the total light a source emits, candela the intensity in a direction, lux the light arriving on a surface, and nits the brightness of a surface or display. Using them with a camera exposure makes lighting behave like real photography, so a lamp, the sun and the sky can all be balanced in the same system. Unreal Engine mainly uses candela, lumens and EV values.
In production
Learn a few reference values: the sun is about 100,000 lux at noon, a lit office about 500 lux and a candle about 1 candela. Set the camera exposure to match the scene, and the lights then look right without arbitrary numbers. If your lights seem way too bright or dark, a mismatch between light units and exposure is the likely cause. Stylized projects often prefer simple unitless values.
